The present invention relates generally to the maintenance of electric cables and, more particularly, to a non-invasive device and method for detecting energization of electric conductors. The device is particularly useful for testing underground electric power distribution cables. However, it should be recognized that the device of the present invention may be used to test overhead electric power cables as well as other types of electric cables that produce a sufficient electric field when energized.
It is often necessary to determine the energization status of an electric cable prior to performing any maintenance. Failure to correctly determine whether an electric cable is live or dead can be dangerous and costly. At a minimum, the mistake can result in the need for additional repairs to the electric cable. At worst, the mistake presents a significant hazard to the safety of the maintenance workers. If the maintenance workers are operating under the mistaken belief that a live electric cable is dead, there is a considerable risk of physical injury or electrocution.
A power outage is one example of a situation that requires a determination of the energization status of an electric cable. The cause of a power outage may be a faulted cable. In that event, maintenance workers are typically called to the site to locate the dead cable. In many cases, the dead cable is surrounded by a plurality of live cables. If the cables are underground, a trench is typically dug to uncover the cables. Thereafter, different methods have been used for testing the energization status of the cables to identify the dead cable. However, these methods are either too time consuming, destructive, dangerous, or unreliable.
One common method is to pierce the cable with a spiked penetrator clamp. This requires piercing the cable insulation with at least one spike to make electrical contact with the internal conductor. In the event that the conductor is energized, a rather large electrical discharge results. The electrical discharge endangers anyone who is in the vicinity. Moreover, this testing method is destructive and unreliable. The damage caused by piercing the cable will have to be repaired, or the cable will have to be replaced. In addition, a false indication, i.e., no electrical discharge, occurs if the spike(s) do not make contact with the energized conductor.
Another method is to inject test signals into the electric cable. For example, probes are advanced into the cable to contact the screen layer and/or the shield of the cable. Test signals are then input and measurements are taken to determine the energization status of the cable. However, this method causes damage to the cable which must eventually be repaired, or the cable may have to be replaced. Furthermore, the method can be time consuming, and the test equipment can be expensive.
Still another method is to measure the magnetic field emanating from an electric cable. This method may not require the electric cable to be pierced. However, this method can be unreliable. A magnetic field sensor may lose sensitivity near the middle of the length of an energized electric cable. As a result, testing the energized electric cable at the middle of its length may result in a false negative reading. Additionally, the strength of a magnetic field is directly related to the amount of current flowing through the conductor, i.e., more current produces a stronger magnetic field. However, the voltage carried by the conductor can be very high even though the current flowing through the conductor is very low. As a result, a magnetic field sensor may also provide a false reading when testing a conductor carrying a high voltage if the current is low.
Measuring the surface temperature of an electric cable is another testing method that may not require the cable to be pierced. However, this method may also be unreliable. The cable temperature is proportional to load current. If the cable does not carry any load, its temperature would be roughly at ambient, yet it could still be energized. In addition, variations in the ambient temperature may lead to inaccurate readings. Moreover, these testing devices can be costly.
In light of the shortcomings of the foregoing testing methods and devices, a need exists for a testing method and device that provides accurate readings without the need to pierce the electric cable or to expose the internal conductor. Another need exists for a testing method and device that is safe and does not cause an electrical discharge when the electric cable is live. Still another need exists for a testing method and device that does not require signals to be injected into the electric cable. There is also a need for a testing method and device that does not require expensive testing equipment. In addition, a need exists for a testing method and device that is accurate over the entire length of the electric cable. Finally, a need exists for a testing method and device that is dependent on the voltage, as opposed to the current, carried by the conductor.
The present invention satisfies some or all of these needs. One embodiment of the present invention is a device for detecting energization of an electric conductor. The electric conductor may be an internal part of an insulated electric cable having an outer jacket, e.g., a URD cable. The device includes a sensor for detecting an electric field produced by the electric conductor when it is energized. The sensor is adapted to produce a signal in response to the electric field. An amplifier is in electrical communication with the sensor, and it is adapted to amplify the signal a desired amount. A detector circuit comprising at least one comparator is in electrical communication with the amplifier. The detector circuit is adapted to compare an output of the amplifier to a threshold value to determine if the electric conductor is energized. For example, the threshold value may be in the range of from about 0.5 to about 5 volts.
In an optional embodiment of the present invention, the sensor is adapted to extend substantially around the electric conductor or, in the case of an electric cable having an outer jacket, the outer jacket. The sensor may be a capacitor comprising a first plate, a second plate, and a dielectric connecting the first plate and the second plate. If the electric conductor is not enclosed by an outer jacket, the sensor can detect the electric field when the first plate is placed sufficiently near the electric conductor and the second plate is grounded. Similarly, when the electric conductor is substantially enclosed by an outer jacket, the sensor can detect the electric field when the first plate is placed sufficiently near or substantially against the outer jacket and the second plate is grounded.
Optionally, the sensor further comprises a second dielectric connected to the first plate such that the first plate is positioned between the first dielectric and the second dielectric. In such an embodiment, the sensor is adapted to detect the electric field when the second dielectric is placed sufficiently near or substantially against the electric conductor or, in the case of an electric cable having an outer jacket, the outer jacket. In addition, the sensor may also comprise a shield and a third dielectric, wherein the third dielectric connects the shield to the second plate.
As is known in the art, the gain of the amplifier may be comprised of a plurality of amplification stages. The gain of the amplifier is preferably adjustable. An adjustable gain preferably enables the same device to be used to test conductors carrying different voltages. In one example, the amplifier has a gain of up to about 80 decibels (dB). Nevertheless, it is appreciated that the amplifier may have a gain greater than about 80 dB.
The detector circuit may include at least one light-emitting device, e.g., a light-emitting diode, in electrical communication with a comparator to indicate the status of the electric conductor. For example, the detector circuit may include one comparator in electrical communication with a light-emitting device such that the light-emitting device emits light when the electric conductor is energized. In addition, the detector circuit may include a second comparator in electrical communication with a second light-emitting device such that the second light-emitting device emits light when the electric conductor is not energized.
The device preferably includes at least one filter before the amplifier and/or interposed between amplification stages and/or interposed between the amplifier and the detector circuit. At least one filter is preferably adapted to filter the signal to improve the performance of the device. The type of filter may be selected based on the frequency of the signal that will be carried by the electric conductor when it is energized. For example, the filter may be a 60 Hz bandpass filter if the electric conductor is intended to transmit a 60 Hz power signal. For another example, the filter may be a 50 Hz bandpass filter if the electric conductor is intended to transmit a 50 Hz power signal.
The sensor may purposefully or inadvertently sense an excessive signal. Accordingly, the device may also include a voltage or current surge protection circuit in electrical communication with the amplifier. The surge protection circuit is preferably adapted to protect the amplifier and the detector circuit from signals that exceed a predetermined voltage or current level.
The device may include an insulated handle connected to the sensor. The insulated handle preferably enables an operator to place the sensor in position to measure the electric field. An example of an insulated handle is a commercially available hotstick.
Another embodiment of the present invention is a system for detecting energization of an electric cable. The system may include any of the optional or preferred features of the above-described device of the present invention. The system includes an electric cable that is adapted to produce an electric field when energized. The electric cable comprises an electric conductor. The system also includes a sensor adapted to produce a signal in response to the electric field. The sensor comprises a first plate, a second plate, and a dielectric connecting the first plate and the second plate. The first plate substantially abuts the electric cable, and the second plate is connected to ground. An amplifier is in electrical communication with the sensor. The amplifier is adapted to amplify the signal a desired amount. A detector circuit is in electrical communication with the amplifier. The detector circuit includes at least one comparator and at least one light-emitting device. The detector circuit is adapted to compare an output of the amplifier to a threshold value to determine if the electric conductor is energized. The at least one light-emitting device is in electrical communication with the at least one comparator, and it is adapted to indicate the status of the electric conductor.
The present invention also includes a method for detecting energization of an electric cable. The method begins by providing an electric conductor adapted to produce an electric field when energized. Also provided is a sensor adapted to detect the electric field. The sensor is further adapted to produce a signal in response to the electric field. The sensor detects the electric field, and the signal is processed to determine whether the electric conductor is energized.
The method may include any of the optional or preferred features of the aforementioned embodiments of the present invention. The processing may be performed with any circuitry including, but not limited to, analog circuitry, digital circuitry, digital signal processing circuitry, software, other suitable, conventional, or similar types of electronic circuitry, and combinations of any of these types of circuitry. During processing, the signal may be amplified a desired amount. In addition, the processing step may include filtering the signal. For example, the signal may be filtered at least before each amplification stage.
Processing may also include comparing the signal to a threshold value to determine the energization status of the electric conductor. The comparison may occur after the signal is adequately amplified and filtered. One example of the comparison step includes providing the signal to a meter having a display such that an operator can compare the reading of the display to a threshold value. Another example of the comparison step includes causing a light-emitting device to emit light if the signal exceeds a threshold value. Moreover, it may further include causing a second light-emitting device to emit light if the signal does not exceed a threshold value. In this or a similar manner, the light-emitting device(s), e.g., light-emitting diode(s), can provide a visual indication of the status of the internal electric conductor.